Patients with congenital adrenal hyperplasia attributable to 21-hydroxylase deficiency are treated with glucocorticoids. Glucocorticoid administration, even in substitution doses, may cause decreased bone mineral density (BMD) and obesity. The purpose of this study was to determine BMD, lean mass, and fat mass in young adult male (M, n = 15) and female (F, n = 15) patients with 21-hydroxylase deficiency, who had been treated with currently recommended low doses of glucocorticoids. Measurements were performed with dual-x-ray absorptiometry. In addition, calcaneal ultrasound measurements were performed (broadband ultrasound attenuation and speed of sound). Results were compared with those in age- and sex-matched controls; to adjust for height, lean and fat mass were divided by (height)(2). M and F patients [M, 21.7 +/- 2.4; F, 20.6 +/- 2.9 yr old (mean +/- SD)] were shorter than the controls (M, P < 0.001; F, P < 0.003) and their body mass indices were higher [M patients (25.0 +/- 3.6) vs. controls (22.3 +/- 1.9 kg/m(2)) (P < 0.02); F patients (25.5 +/- 4.5) vs. controls (21.9 +/- 2.3 kg/m(2)) (P < 0.02)]. BMD values (lumbar spine L1-L4, femoral neck, and total body) were not different from controls. Calcaneal ultrasound measurements showed that M patients had higher speed of sound values [M patients (1564 +/- 38) vs. controls (1529 +/- 29 m/sec) (P < 0.01)]. Lean mass in M and F patients was not different from controls when adjusted for height. Fat mass was higher in M and F patients when adjusted for height [M patients 5.6 +/- 2.9 vs. controls 2.7 median (1.7-7.0 min-max) kg/m(2) (P < 0.04); F patients 8.7 +/- 2.8 vs. controls 5.8 (4.3-10.7) kg/m(2) (P < 0.02)]. Relative fat mass (fat mass as a percentage of the total body mass) was higher in patients, compared with controls [M patients 22.0 +/- 9.1 vs. controls 12.8 (8.5-27.0)% (P < 0.04); F patients 34.1 +/- 5.0 vs. controls 29.0 +/- 5.1% (P < 0.02)]; this resulted from increased fat mass, not from decreased lean mass. Fat distribution over the body was not different in patients and controls. No significant correlations were found between cumulative glucocorticoid doses in the last 0.5, 2, or 5 yr or mean salivary morning levels of 17-hydroxyprogesterone and androstenedione in the last 5 yr on one hand and bone parameters, lean mass, or fat mass on the other hand. We conclude that, at prevailing low-dose glucocorticoid regimens, young adult patients with 21-hydroxylase deficiency have normal BMD. Their lean mass is in accordance with height, but fat mass is increased, with a normal distribution over the body. This results in a higher fat percentage of the total body and a higher body mass index than in healthy peers. Because overweight and increased fat mass are associated with the metabolic syndrome and increased cardiovascular risk, weight management should have appropriate attention in the follow-up of congenital adrenal hyperplasia patients, to prevent overweight-associated morbidity.